This invention relates to a turbomolecular vacuum pump which has a vertically oriented rotor supported on a vertically upwardly extending stub shaft affixed to the pump housing. The bearing arrangement of the rotor is situated within the prevacuum chamber surrounded by hollow rotor extends.
For generating a high vacuum that is free from hydrocarbons, several types of turbomolecular vacuum pumps are known. By means of these pumps there may be achieved pressures in the molecular pressure range, that is, approximately between 10.sup..sup.-2 and 10.sup..sup.-9 Torr. In turbomolecular vacuum pumps of this type, particular difficulties have been encountered in the design of the rotor bearing, because of the high-speed rotation amounting to approximately 20,000 rpm.
In known turbomolecular vacuum pumps the rotor is supported on a vertically upwardly oriented stub shaft which is affixed to the housing and which includes oil-lubricated roller bearings with a lubricant (oil) circuit. The bearing arrangement is disposed in the prevacuum chamber of the pump to ensure that during operation, the light fractions of hydrocarbons escaping from the oil because of the oil flow from the high vacuum part to the prevacuum part of the pump, do not enter the high vacuum chamber. During standstill of such a turbomolecular vacuum pump, however, there is no directed flow which would prevent the return of the hydrocarbons into the high vacuum chamber. Thus the undesirable possibility is present that these hydrocarbons are introduced by diffusion from the prevacuum chamber into the high vacuum chamber. The penetration of the hydrocarbons into the vacuum chamber and thus, at the same time, their penetration into the high vacuum receptacles connected with the high vacuum chamber of the turbomolecular pump must not be permitted in many cases, particularly when the pump is used in physical research or the like. Consequently, the use of such known turbomolecular vacuum pumps is limited.
For avoiding the above-outlined disadvantages, turbomolecular vacuum pumps have been developed in which, by using a lubricating oil having a particularly low vapor pressure, a hydrocarbon diffusion from the roller bearing is largely prevented. The suppression of back-flow of the hydrocarbons into the high vacuum chamber is, however, insufficient for some applications.
It is further in the domain of the prior art to provide a lubricant-free magnetic bearing for the rotor in which a magnetic axial bearing is situated in the high vacuum chamber and in the suction chamber of the turbomolecular pump, thus constricting the free suction section of the pump. For a radial support of the stator, there is disposed a radial bearing beneath the turbine formed of a rotor and a stator. Although such an arrangement results in a hydrocarbon-free high vacuum, the provision of a magnetic axial bearing at the high vacuum side proved to be disadvantageous in connection with the supply of electric energy, the heatup of the coils and thus the provision of the possibility of release of gases and the like. It was further found that the arrangement of an upper axial bearing in connection with only one radial bearing disposed beneath the turbine cannot ensure all degrees of freedom for the rotor. The known magnetic bearing thus displays, besides disadvantages relating to vacuum technology, instabilities of rotor motion including precessional and nutational motions. Further, this known arrangement does not provide a satisfactory emergency bearing. The latter should be a mechanical bearing (as opposed to a magnetic bearing), expediently in the form of a slide bearing which, in case of a malfunctioning of the magnetic bearing (caused, for example, by power failure), ensures a smooth stoppage of the rotor without the danger of damage or destruction.